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Abstract:

Methods and systems for determining coil eccentricity of seismic sensors
configured or designed for use in seismic signal detection. A direct
electrical current is applied to a moving coil of a seismic sensor such
that the moving coil is dislocated from a neutral position relative to
the magnetic field in the seismic sensor. A predetermined indicator is
measured and eccentricity of the coil (δ) relative to the center of
the magnetic filed is determined using the predetermined indicator.

Claims:

1. A system for determining coil eccentricity of one or more seismic
sensor configured or designed for use in seismic signal detection,
comprising: at least one seismic sensor, comprising: a housing; at least
one magnet for creating a magnetic field mounted within the housing; a
moving coil mounted within the housing; at least one spring assembly
connected to the housing and the moving coil for supporting the moving
coil for transduction within the magnetic field; a digital signal
processor in communication with the seismic sensor; a set of instructions
executable by the processor that, when executed: applies to the moving
coil of the at least one seismic sensor a direct electrical current such
that the moving coil is dislocated from a neutral position relative to
the magnetic field in the seismic sensor; monitors a predetermined
indicator relative to the dislocation of the coil from the neutral
position; and determines eccentricity of the coil (δ) relative to
the center of the magnetic filed using the predetermined indicator.

2. A system for determining coil eccentricity of one or more seismic
sensor according to claim 1, wherein the at least one seismic sensor is
configured for positioning on one of on land, within a borehole and at a
seabed.

3. A system for determining coil eccentricity of one or more seismic
sensor according to claim 1, further comprising an adjustment mechanism
for adjusting or correcting the eccentricity of the moving coil.

4. A system for determining coil eccentricity of one or more seismic
sensor according to claim 1, wherein the set of instructions, when
executed, further performs a step test after dislocating the moving coil
from the neutral position.

5. A system for determining coil eccentricity of one or more seismic
sensor according to claim 1, wherein the system is configured or designed
for quality control (QC) activities during manufacture of the at least
one seismic sensor.

6. A system for determining coil eccentricity of one or more seismic
sensor according to claim 1, wherein the system is configured or designed
for monitoring one or more of: an amplitude of a response signal of the
at least one seismic sensor based on natural oscillation of the moving
coil of the seismic sensor; a sound caused by the moving coil colliding
with an end of the seismic sensor; a waveform that is generated by
contact of the moving coil with an end of the seismic sensor; and total
harmonic distortion (THD) of a signal output from the at least one
seismic sensor.

Description:

RELATED APPLICATIONS

[0001] This is a divisional application of currently co-pending U.S.
patent application Ser. No. 12/180,560, filed Jul. 27, 2008, the contents
of which are herein incorporated by reference.

TECHNICAL FIELD

[0002] The present invention relates to devices for sensing vibrations in
earth formations. More specifically, the present disclosure is directed
to electrodynamic sensing devices, such as geophones and seismometers,
that have a moving coil placed in a magnetic field in a centered
position. The present disclosure may be applicable to other types of
vibration transducers, either in sensing or transmitting operation.

BACKGROUND

[0003] In the oil and gas industry seismic sensors are deployed at various
locations, such as on the earth surface, in the sea, at the seabed, or in
a borehole, to provide operationally significant subsurface structural
and material information by measuring seismic signals reflected from
changes in the subsurface structures. In this, seismic sensors are
commonly used for purposes of obtaining useful data relating to acoustic
impedance contrasts in subsurface structures.

[0004] In seismic signal detection, the vibrations in the earth resulting
from a source of seismic energy are sensed at discrete locations by
sensors, and the output of the sensors used to determine the structure of
the underground formations. The source of seismic energy can be natural,
such as earthquakes and other tectonic activity, subsidence, volcanic
activity or the like, or man-made such as acoustic signals from surface
or underground operations, or from deliberate operation of seismic
sources at the surface or underground. For example, the sensed seismic
signals may be direct signals that are derived from micro-seismicity
induced by fracturing or reservoir collapse or alteration, or reflected
signals that are derived from an artificial source of energy. Sensors
fall into two main categories; hydrophones which sense the pressure field
resulting from a seismic source, or geophones which sense vibration
arising from a seismic source.

[0005] Typically, geophones are sensitive to vibrations of low or very low
frequency. As depicted in FIG. 1B, a typical geophone 10 has one or more
cylindrical moving coil 12 that is suspended by springs 20, 22 so as to
be disposed around a magnet 15 having pole pieces 16, 18. The geophone 10
has a housing 24 and end caps 26. Each moving coil 12 is maintained at a
neutral, rest position by the springs 20, 22, and is free to oscillate in
a magnetic field of the magnet 15 from a centered position thereof.
Springs 20, 22 are designed to maintain the coil 12 at a centered,
equilibrium position relative to the magnetic field of the magnet 15.
Note again FIG. 1B.

[0006] When the earth moves due to the seismic energy propagating either
directly from the source or via an underground reflector, the geophone,
which can be located at the earth's surface, in the sea or at the seabed,
or on the wall of a borehole which penetrates the earth, moves with the
particle motion caused by acoustic wave propagation. If the axis of the
geophone is aligned with the direction of motion, however, the moving
coil mounted on the spring inside the geophone stays in the same position
causing relative motion of the coil with respect to the housing. When the
coil moves in the magnetic field, a voltage is induced in the coil which
can be output as a signal.

[0007] If the geophone is tilted, i.e., is moved away from the orientation
that it is designed for, the moving coil is eccentered with respect to
the magnetic field in the magnet. Note FIG. 1C. For example, as depicted
in FIG. 1A, typically vertical geophones are used in land seismic survey
operations. The spring to support the moving coil is pre-stressed to
compensate for gravitational force so that the moving coil is centered in
the geophone. However, the geophones are manually planted in the ground
and may not be vertical. If such a geophone is tilted, the pre-stressed
spring causes the moving coil to move in the upward direction relative to
the neutral position of the coil in the vertical position of the
geophone, as depicted in FIG. 1C. The neutral or rest position of the
moving coil is designated in FIG. 1C as x0, and the displaced
position due to tilt θ is designated as x. Eventually, if the
amount of tilt is large, the moving coil hits an end cap of the geophone
so that the geophone is no longer able to respond to the seismic
vibrations.

[0008] Although FIG. 1A depicts exemplary land seismic with typical
vertical geophones, it is also possible to use three-component geophones
of the type discussed herein in connection with seabed and borehole
seismic.

[0009]FIG. 1D illustrates graphically the relationships between tilt of a
10 Hz vertical geophone and the geophone response parameters So,
Do, and fo, using measured data. In this, as evident from FIG.
1D, if a vertical geophone is tilted from its vertical position the
geophone response parameters So, Do, and fo change based
on the amount of tilt.

[0010] In land seismic survey operations, seismic data are processed by
assuming that all the geophones that are planted on the land surface are
vertical. If seismic waves propagate in the upward direction, a tilted
geophone will output signal that is altered due to tilt and reduced by an
amount equal to cos(θ), where θ is measured from
vertical--note FIG. 1C. As a consequence, incorrect orientation of the
geophones can cause misinterpretation of the formation properties, by
changing the apparent amplitude of reflected waves.

[0011] In seabed seismic survey operations, an ocean bottom cable (OBC) is
deployed from a boat to the seabed. Note FIGS. 2A and 2B. Seismic sensors
are mounted on the side of the cable. After the cable is deployed in the
sea, the orientation of the sensors may be horizontal, vertical or upside
down. Gimbaled mountings may be used so that a sensor is always
vertically oriented irrespective of how the sensor is deployed. Recently,
three-component omni-tiltable geophones have been used in sensor packages
with a tiltmeter or inclinometer. By knowing the orientation of the
mounting of the three-component geophone, it is possible to rotate the
axis of the seismic measurements. Ideally, a magnetometer may be employed
to know the horizontal orientation of the sensor package so as to
transpose the detected seismic signals in the physical earth coordinates
to determine from which direction the seismic signals arrive at the
seismic sensors.

[0012] In a borehole seismic survey, one or more geophone is deployed
downhole in a borehole. Note FIGS. 3A and 3B. The trajectory of the
borehole is usually known by independent measurement. If the length of
the deploying cable is known, i.e., the along depth, it is possible to
determine orientation and position of the downhole sensor package, i.e.,
azimuth, inclination, depth and horizontal departure from the well head.
The information missing is the relative bearing of the sensor package. As
used herein, "relative bearing" refers to the angle of sensor package
orientation. The sensor package or sonde is typically cylindrical and may
rotate in the borehole, and the orientation of the two horizontal
geophones will not be known. To identify the sensor orientation, it may
be possible to deploy such downhole geophones with a gyroscope.
Alternatively, a tiltmeter or inclinometer may be integrated with the
downhole geophones to determine relative geophone bearing against the
direction of gravity.

[0013] In addition to the issues discussed above, others arise during
manufacturing and assembly of geophones. In this, during manufacture the
geophone moving coil may not be properly centered around the magnetic
field in the magnet. After assembly, it is not possible to see whether or
not the moving coil is properly centered around the magnet so as to be at
its desired neutral rest position.

[0014] Displacement of the moving coil from its neutral rest position
during assembly of the geophone may lead to changes in the geophone
response parameters and increase in harmonic distortion. The offset of
the coil reduces the dynamic range of the geophone. In a worst case, the
geophone moving coil may hit the top or the bottom of the housing and
therefore not respond to seismic signals that are received by the
geophone. In particular, a properly centered moving coil is highly
important for low frequency geophones, such as seismometers, since the
acceptable operating tilt range for such geophones is small, i.e., in the
order of a few degrees. Such low frequency geophones or seismometers
often use a built-in carpenter's level or eye bubble to guide
installation of the devices; however, such eye bubble levels show only
the tilt angle of the geophone or seismometer housing relative to
gravity, but do not show the eccentricity of the moving coil without a
built-in displacement sensor inside the geophone or seismometer.

[0015] As previously mentioned, in the past, gimbaled geophones have been
employed to avoid tilt in the geophone. However, gimbaled geophones tend
to be bulky and are more expensive due to the additional hardware that is
required for the gimbaled structure. Geophones with tiltmeters and other
tilt determining sensors are known in the art, but require additional
hardware and are difficult to fit in the limited space that is typically
available in seismic surveying operations. In addition, extra wiring is
required for electrical connection. Since a geophone type device is a
passive sensor, only a twisted wire cable is required to connect the
geophone to a data acquisition system. Typically, in land, seabed, or
borehole seismic acquisition operations many geophones are connected
using multi-twisted pair cables. Extra wiring for built-in tilt sensors
means that additional conductors must be added to the cables thereby
increasing cable weight and cost, and the maintenance costs for the
cables. In addition, larger connectors are required which poses a
reliability issue. For seabed and borehole operations, any additional
connectors or connections to the cable are perceived as unreliable due to
a tendency to leak. Therefore, increased wiring is not a preferred
approach in seismic operations.

[0016] For single seismic sensors having tilt accelerometers, the
electronics may be located away from the sensors causing alignment errors
between the seismic sensors and the tilt accelerometers. Such errors are
difficult to control making the use of such configurations problematic.

[0017] Accordingly, it will be appreciated that there exists a desire to
improve upon conventional methods and systems that use geophones in order
to improve the accuracy of seismic measurements.

[0018] The limitations of conventional seismic sensor designs noted in the
preceding are not intended to be exhaustive but rather are among many
which may tend to reduce the effectiveness of previously known sensor
mechanisms in field operation. The above should be sufficient, however,
to demonstrate that sensor structures existing in the past will admit to
worthwhile improvement.

SUMMARY

[0019] Embodiments disclosed herein provide methods and systems for
seismic sensors, such as geophones and seismometers. In particular, some
embodiments of the present disclosure provide methods and systems for
determining eccentricity (δ) of a moving coil of a seismic sensor
that is designed or configured for seismic signal detection.

[0020] In certain embodiments herein, the techniques of the present
disclosure may be utilized for determining tilt of a geophone, for
example, a geophone that is planted in the ground for land seismic. In
yet other embodiments herein, quality control (QC) of geophones planted
in the ground may be performed based on the determined eccentricity of
the moving coils of the geophones to see if the geophones are vertically
planted within the tolerance for seismic signal acquisition by use of the
geophones. In such a situation, it would be possible to inform the person
or persons who planted the geophones to replant them so that the
geophones are properly planted in the ground for purposes of land seismic
survey. The present disclosure contemplates that the person or persons
who planted the geophones may perform the test to make sure that the
geophones are properly planted. Such QC information may be provided to a
client as evidence of the quality of the geophone planting operation. QC
information of this type provides a unique answer product/service that
could be of high value to a client.

[0021] In yet other embodiments herein, relative bearing of a seismic
sensor package deployed in a borehole or at a surface of a seabed may be
determined based on the tilt of the geophone(s) in the sensor package.

[0022] In yet other embodiments herein, eccentricity of the moving coil
may be utilized for purposes of adjusting assembly parameters for
geophones, such as adjusting an assembly jig, so that the moving coils of
the geophones are properly centered in the magnetic fields during
manufacturing. By measuring the eccentricity of the moving coils of the
geophones after fabrication, poorly assembled geophones may be rejected
and only the correctly assembled geophones provided to the customer
thereby eliminating defective/improperly functioning geophones during the
manufacturing process. As a further aspect of the present disclosure,
adjustment mechanisms and techniques are provided to adjust a moving coil
of a geophone to compensate for or correct geophone coil eccentricity.
Such mechanisms may be installed in geophones so that coil eccentricity
may be easily corrected on an as needed basis.

[0023] In further embodiments disclosed herein, an alternating electrical
current is superimposed on a direct electrical current applied to the
moving coil of a geophone; the applied direct electrical current is
changed such that the moving coil hits either the top or the bottom cap;
and a top or bottom of the geophone housing is determined based on the
level of the direct electrical current at which distortion due to the
moving coil collision appears in the sinusoidal current. It may also be
possible to hear the sound of the coil hitting the top or bottom end cap
of the geophone. Furthermore, electrical circuitry may be provided in the
geophone so that contact between the moving coil and the end caps of the
geophone causes an electrical leakage that is monitored. One or more
geophone assembly parameter may be adjusted to correct the coil
eccentricity.

[0024] In some embodiments of the present disclosure, a method for
determining coil eccentricity of a seismic sensor configured or designed
for use in seismic signal detection comprises applying to a moving coil
of the seismic sensor a direct electrical current such that the moving
coil is dislocated from a neutral position relative to a magnetic field
in the seismic sensor; monitoring a predetermined indicator relative to
the dislocation of the coil from the neutral position; and determining
eccentricity of the coil (δ) relative to the center of the magnetic
field using the predetermined indicator. In some aspects disclosed
herein, the method further comprises performing a step test after
dislocating the moving coil from the neutral position. The seismic sensor
may be vertically oriented and the step test may be performed by
releasing the coil from an upward position relative to the neutral
position; releasing the coil from a downward position relative to the
neutral position; or releasing the coil from both an upward position and
a downward position relative to the neutral position, and combining the
results.

[0025] The step test may be performed during quality control (QC)
activities. In some aspects the method includes releasing the direct
electrical current applied to the moving coil of the seismic sensor,
wherein the predetermined indicator comprises an amplitude of a response
signal of the seismic sensor based on natural vibration of the seismic
sensor. In other aspects herein, the method includes applying the direct
electrical current to the moving coil of the seismic sensor such that the
moving coil collides with an end of the seismic sensor, wherein the
predetermined indicator comprises a sound caused by the moving coil
colliding with the end of the seismic sensor and/or a waveform that is
generated by the contact of the moving coil with the end of the seismic
sensor. In yet other aspects herein, the predetermined indicator
comprises total harmonic distortion (THD) of a signal output from the
seismic sensor.

[0026] In some embodiments, the method includes superimposing an
alternating electrical current on the direct electrical current applied
to the moving coil of the seismic sensor; changing the applied direct
electrical current such that the total harmonic distortion (THD) of the
alternating electrical current increases; and determining a top or bottom
of the seismic sensor housing based on the direct electrical current at
which there is a sudden increase in the THD of the alternating electrical
current. In aspects of the present disclosure, a seismic sensor assembly
parameter may be adjusted to correct the coil eccentricity.

[0027] Some embodiments disclosed herein include a method for detecting
seismic signals comprising deploying one or more seismic sensor
configured or designed for seismic signal detection; applying to a moving
coil of the one or more seismic sensor a direct electrical current such
that the moving coil is dislocated from a neutral position relative to a
magnetic field in the one or more seismic sensor; monitoring a
predetermined indicator relative to the dislocation of the coil from the
neutral position; determining eccentricity of the coil (δ) relative
to the center of the magnetic field using the predetermined indicator;
utilizing the coil eccentricity for determining tilt of the one or more
seismic sensor; and utilizing the tilt of the one or more seismic sensor
for seismic signal detection.

[0028] In some embodiments, a step test is performed in-situ during
seismic signal detection. The step test may be performed during land
seismic survey activities. The step test may be performed during borehole
or seabed seismic survey activities. The step test may be performed
in-situ during seismic survey activities using a wireline survey system.
In some aspects of the present disclosure, tilt of one or more seismic
sensor may be determined during land seismic survey activities. In other
aspects, relative bearing of one or more seismic sensor package may be
determined during borehole or seabed seismic survey activities.

[0029] In aspects herein, the method may include calibrating the one or
more seismic sensor. In other aspects herein, the method may include
calibrating the one or more seismic sensor to determine one or more of DC
resistance (DCR), natural frequency (f0), open circuit sensitivity
(S0), and open circuit damping (D0) of the one or more seismic
sensor.

[0030] Aspects herein provide a system for determining coil eccentricity
of one or more seismic sensor configured or designed for use in seismic
signal detection. The system includes at least one seismic sensor having
a housing; at least one magnet for creating a magnetic field mounted
within the housing; a moving coil mounted within the housing; at least
one spring assembly connected to the housing and the moving coil for
supporting the moving coil for transduction within the magnetic field.
The system also includes a digital signal processor in communication with
the seismic sensor and a set of instructions executable by the processor
that, when executed, applies to the moving coil of the at least one
seismic sensor a direct electrical current such that the moving coil is
dislocated from a neutral position relative to the magnetic field in the
seismic sensor; monitors a predetermined indicator relative to the
dislocation of the coil from the neutral position; and determines
eccentricity of the coil (δ) relative to the center of the magnetic
filed using the predetermined indicator.

[0031] In aspects herein, the at least one seismic sensor may be
configured for positioning within a borehole or at a seabed. In other
aspects of the present disclosure, the at least one seismic sensor may be
configured for positioning on land. The system may include an adjustment
mechanism for adjusting or correcting the eccentricity of the moving
coil. The set of instructions, when executed, may further perform a step
test after dislocating the moving coil from the neutral position. The
system may be configured or designed for quality control (QC) activities
during manufacture of the at least one seismic sensor.

[0032] In other embodiments disclosed herein, the system may be configured
or designed for monitoring one or more of: an amplitude of a response
signal of the at least one seismic sensor based on natural oscillation of
the moving coil inside the seismic sensor; a sound caused by the moving
coil colliding with an end of the seismic sensor; a waveform that is
generated by contact of the moving coil with an end of the seismic
sensor; and total harmonic distortion (THD) of a signal output from the
at least one seismic sensor. In aspects of the present disclosure, the
system may be configured or designed for land seismic survey activities.
In other aspects herein, the system is configured or designed for
borehole or seabed seismic survey activities. In yet other aspects, the
system may comprise a wireline survey system.

THE DRAWINGS

[0033] Other aspects of the present disclosure will become apparent from
the following detailed description of embodiments thereof taken in
conjunction with the accompanying drawings wherein:

[0034]FIG. 1A depicts one exemplary operating context in accordance with
the present disclosure for land seismic survey;

[0035] FIG. 1B is a schematic view of a conventional geophone;

[0036]FIG. 1C illustrates schematically the displacement of the center of
a moving coil of a geophone from its neutral or rest position x0 to
a displaced position x due to tilt θ;

[0037]FIG. 1D depicts the relationships between tilt of a 10 Hz vertical
geophone and the geophone response parameters So, Do, and
fo using actual data;

[0038] FIGS. 2A and 2B depict another exemplary operating context in
accordance with the present disclosure for seabed seismic;

[0039] FIGS. 3A and 3B depict yet another exemplary operating context in
accordance with the present disclosure for borehole seismic;

[0040]FIG. 4A is a flowchart depicting some techniques according to the
present disclosure;

[0041]FIG. 4B is a schematic depiction of one exemplary method and
apparatus for determining center of a moving coil of a geophone;

[0042]FIG. 5 is a schematic representation of one technique according to
the present disclosure for determining geophone coil eccentricity;

[0043]FIG. 6 is a graphical representation of a simulated geophone coil
response in a step test;

[0044]FIG. 7 is a graphical representation of the temperature dependence
of geophone response parameters So, Do, and fo, for a test
geophone;

[0045]FIG. 8 is a graphical representation of simulated geophone coil
responses in step up and step down tests when the geophone is in a
vertical position without pre-stress in the spring;

[0046]FIG. 9 is a schematic representation of a geophone imaginary short
circuit for a geophone;

[0047] FIG. 10 graphically illustrates simulated geophone coil responses
in step up and step down tests with over damping using the imaginary
short circuit shown in FIG. 9;

[0048] FIG. 11 graphically illustrates collision of a moving coil at the
bottom during a step test;

[0049]FIG. 12 is a graphical illustration of exemplary measured data for
geophone coil responses in step up and step down tests;

[0050]FIG. 13 is a graphical illustration of exemplary measured data for
geophone coil responses in step up and step down tests at different tilt
angles;

[0051] FIGS. 14A and 14B depict some exemplary techniques in accordance
with the present disclosure for land seismic survey;

[0052] FIGS. 15A-15C depict some exemplary techniques in accordance with
the present disclosure for seabed and borehole seismic;

[0053] FIGS. 16A and 16B depict some exemplary techniques in accordance
with the present disclosure for determining relative bearing of sensor
packages in seabed and borehole seismic, respectively;

[0054]FIG. 17 is a schematic depiction of one exemplary technique for
adjusting the position of a moving coil of a geophone to correct for coil
eccentricity; and

[0055] FIG. 18 is a schematic depiction of an exemplary network device for
the methods and systems according to the present disclosure.

DETAILED DESCRIPTION

[0056] Turning now to the drawings, wherein like numerals indicate like
parts, the disclosure herein is directed to the concept of eccentricity
(δ) of a moving coil of a geophone type electrodynamic sensor. As
used herein, "eccentricity or displacement of the moving coil" refers to
the deviation or dislocation of the center of the moving coil relative to
the center of the magnetic flux field that is generated by the magnet
inside the geophone housing, i.e., the distance between the center of the
magnetic field in a geophone and the center of the moving coil. Note
again FIG. 1B. In this, the center of the magnetic field in a geophone
may be determined from modeling of the magnetic flux field, or by
measuring the amount of flux density along the gap between the inside of
the geophone housing and the pole pieces of the geophone. As previously
discussed above, in seismic signal detection it is advantageous to use
geophones with moving coils that are centered relative to the magnetic
field that is generated by a magnet mounted inside the geophone housing.

[0057] The present disclosure provides various techniques that may be
utilized to facilitate and improve seismic signal detection. For example,
assembly parameters during geophone manufacturing may be adjusted to
improve the quality of the geophones. Assembled geophones may be checked
for quality to ensure that they comply with required response
specifications to detect or measure seismic signals. In land seismic
operations, tilt of the geophones planted in the ground may be determined
so that appropriate corrective measures may be employed to obtain
vertically oriented geophones for land seismic surveying. In borehole or
seabed deployed seismic systems, relative bearing of the deployed
geophone sensor packages may be ascertained for purposes of processing
the received seismic signals.

[0058] The present disclosure contemplates applicability of the disclosed
techniques to electrodynamic type sensors, such as a geophone or a
seismometer, that are utilized in the field of seismic prospecting, or of
active or passive monitoring of underground reservoirs. The sensors may
be deployed in exploration and/or production wells that are deviated in
relation to the vertical direction, and comprise multi-component
geophones for detecting components of signals that are received along
three orthogonal axes. In aspects according to the present disclosure,
the seismic sensors may be utilized in wireline systems, land seismic
surveying systems, seabed seismic surveying systems, permanent or other
monitoring systems, including systems for monitoring earthquakes or
micro-seismicity in a reservoir, and in factory-based testing and
assembly systems for geophones. Some principles of the present disclosure
are also described in co-pending, commonly owned, U.S. patent application
Ser. No. 11/733,214, titled "Geophone Calibration Technique", the entire
contents of which are hereby incorporated herein by reference.

[0059] As described in greater detail below, the present disclosure
provides various techniques which may be used to facilitate and improve
seismic signal detection. For example, one aspect of the present
disclosure is directed to a technique for in-situ determination of the
tilt of a geophone. Another aspect of the present disclosure is directed
to a technique for improving the accuracy of geophone measurements.

[0060] The present disclosure contemplates application of the principles
herein to various areas, such as wireline, land seismic, seabed seismic,
permanent or other monitoring, hydro-fracture monitoring, production
logging, among others.

[0061]FIG. 1A is a schematic depiction of one exemplary technique and
system in accordance with the present disclosure for land seismic survey.
In FIG. 1A, a surface seismic source 206, such as a vibrator, is shown
that may be utilized to produce seismic signals in the ground. The
seismic signals are received by geophones 202 that are planted vertically
at the surface, and recorded by any suitable recording unit 214, such as
the mobile recording unit depicted in FIG. 1A. Although FIG. 1A depicts
exemplary land seismic with typical vertical geophones, it is also
possible to use three-component geophones of the type discussed below in
connection with seabed and borehole seismic. In this, it is contemplated
that the techniques disclosed herein with respect to three-component
geophones could be applied to land seismic survey as well.

[0062] In order to gain a better understanding of the various techniques
and features described in this application, a brief description of
geophone measurement techniques will now be provided. A seismic survey
measures seismic waves propagated through the earth to map structural
images in the earth. Geophones are often used to detect seismic signals
at various locations, such as, for example, downhole, at ground surface
and/or at the seabed. An example of a conventional geophone is shown in
FIG. 1B. The geophone 10 of FIG. 1B includes moving coil 12 mounted on a
bobbin 14, a magnet 15, a pair of pole pieces 16, 18 with suspension
springs 20, 22 and a housing 24 as shown in FIG. 1B. The pole pieces 16,
18 and housing 24 are made of magnetically permeable material and form a
magnetic field in which the moving coil 12 is suspended. In the example
of FIG. 1B, the moving coil 12, bobbin 14, and suspension springs 20, 22
collectively form the effective moving mass portion (m) of the geophone.
As used in this application, the term "geophone" is intended to include
conventional-type geophones such as that illustrated in FIG. 1B, and very
low frequency geophones such as seismometer type electrodynamic sensors,
as well as geophone accelerometer (GAC) devices from Schlumberger
Corporation which, for example, may be configured or designed to measure
relatively wider acceleration ranges than conventional-type geophones. As
shown in the embodiment of FIG. 1B, the geophone 10 includes a moving
coil that is suspended in a magnetic flux by means of a spring or a pair
of springs. The moving coil tries to stay in the same position while the
housing of the geophone is moved in response to external vibrations. FIG.
1C is a schematic depiction of the eccentricity in a moving coil of a
geophone that is caused by tilt angle θ, which causes the moving
coil to be displaced from its neutral position x to the position xo
as depicted in FIG. 1C. FIG. 1D shows in graphs the changes in geophone
response parameters So, Do, and fo due to tilt of a 10 Hz
vertical geophone using measured data.

[0063] FIGS. 2A and 2B are schematic depictions of other exemplary
techniques and systems in accordance with the present disclosure for
seabed seismic survey. In FIG. 2A, a sea surface seismic source 206, such
as a gun boat, is shown that may be utilized to produce seismic signals
in the seabed. The seismic signals are received by geophones that are
attached to an ocean bottom cable (OBC) 202, and recorded by any suitable
recording unit 214, such as the recording boat depicted in FIG. 2A. FIG.
2B shows a portion of the OBC with 3-component geophone packages or sonde
(two are shown in FIG. 2B) attached to the cable. Each sensor package may
have three omni-tiltable geophones arranged along the x, y, and z axes
thereof (note FIG. 2A). As indicated in FIG. 2B, one geophone of the
3-component geophone package will be horizontally oriented whereas the
orientation directions of the remaining two geophones will have to be
determined. As described in further detail below (note FIGS. 15A-15C and
16A-B), techniques are provided herein for determining the tilt of the
geophones in the sensor package and the relative bearing of the sensor
package.

[0064]FIG. 3A shows another possible configuration that may be used in
the practice of the techniques described herein. In FIG. 3A, borehole 200
may be a previously drilled well or a borehole that is being drilled. A
seismic source 206 is used to generate a seismic signal 216. The source
206 may be any type of suitable instrumentation for generating the
desired signals. The generated signals 216 propagate through the
formation, and some signals reach the borehole and a sensor section or
sonde 202 of the borehole tool having one or more sensors for detecting
the seismic signals. The sensor section 202 and the associated sensors
may be used as the primary apparatus for collecting the seismic
measurements. In one embodiment, the information collected by the sensor
section 202 may be transmitted uphole via a suitable cable 204, for
example, a wireline, or other conveyance that is configured for data
telemetry, to an analysis module 214 on the surface of the borehole. The
analysis module 214 may be a stand alone, or may be integrated into a
field vehicle as shown in one example of FIG. 3A. Alternatively, or in
combination, some processing or analysis may be conducted downhole, and
processed data may be sent uphole by suitable data telemetry apparatus
for further processing or other purposes as desirable or necessary.

[0065] The sensor section 202 is moved through the borehole 200 by winch
210, via a suitable arrangement in the drilling tower 208, while seismic
signals are detected by sensors in the sensor section 202. A device 212
may be used to record the depth of the sensor section so it is known when
a measurement is taken.

[0066]FIG. 3B shows a portion of the borehole tool in FIG. 3A having the
sensor section or sonde 202 with 3-component geophones (one sensor
section is shown in FIG. 3B). Each sensor section may have three
omni-tiltable geophones arranged along the x, y, and z axes thereof as
illustrated in FIG. 3B. As indicated in FIG. 3B, one geophone of the
3-component geophones will be oriented along the borehole whereas the
orientation directions of the remaining two geophones will have to be
determined (note FIGS. 15A-15C and 16A-B).

[0067]FIG. 4A is a flowchart depicting various techniques disclosed
herein. In accordance with the principles discussed hereinafter, the
moving coil of a geophone is raised and/or lowered (Step 100) by
application of a direct electrical current to the geophone. A
predetermined indicator is monitored (Step 102), which is a result of the
dislocation of the coil. The predetermined indicator relative to the coil
movement may be a sound caused by the moving coil colliding with an end
cap of the geophone. The predetermined indicator relative to the coil
movement may be a waveform that is generated by the collision of the
moving coil with an end cap. The predetermined indicator relative to the
coil movement may be total harmonic distortion (THD) of an alternating
electrical current superimposed on the direct electrical current applied
to the geophone. The predetermined indicator relative to the coil
movement may be a geophone response signal based on natural vibration of
the geophone when the direct electrical current is released.

[0068] In some aspects of the present disclosure, the predetermined
indicator may be the sound that is heard or recorded when the moving coil
of the geophone hits an end cap and/or a sinusoidal wave change that
occurs on collision and is visually noted by an oscilloscope, or by other
techniques. On occurrence of the predetermined indicator of sound and/or
sinusoidal wave change the corresponding applied direct electrical
current is noted and the position x of the moving coil relative to the
neutral position x0 is determined. The present disclosure
contemplates applying AC current to a geophone to vibrate the moving coil
and superimposing DC current to lift and/or lower the coil so that the
moving coil of the geophone hits either the top or the bottom end cap of
the geophone.

[0069] It is assumed that AC current is high frequency and the coil stroke
due to AC current is negligible. For a geophone without pre-stress in the
spring(s), for example, an omni-tiltable geophone, the DC current
balances with gravitational force, mg, and the displacement force of the
spring, kx, as:

S0I=mg+kx

where S0 is open circuit sensitivity of the geophone, I is the DC
current at which the predetermined indicator is noted, m is the mass of
the moving coil, g is the gravitational acceleration, k is the spring
constant, and x is the position of the moving coil relative to the
neutral position or natural displacement due to gravity x0. Note
FIGS. 1C and 5. By noting the current I, and knowing S0, m, and k,
from other sources, x may be determined by:

x = S 0 I - mg k or x = S 0 I m
ω 0 2 - g ω 0 2 ##EQU00001##

where ω0 is the angular natural frequency defined as
ω0=2πf0 and f0 is the natural frequency of the
geophone.

[0070] As discussed above, the predetermined indicator may be sound and/or
waveforms that are observed when the moving coil strikes an end cap.
Furthermore, the predetermined indicator may be a sudden increase in THD
when the moving coil hits the end cap. In addition, the predetermined
indicator may be the amplitude of the natural oscillation of the moving
coil when an applied current is removed.

[0071] Eccentricity of the coil (δ) is determined (Step 104) using
one or more predetermined indicator. As previously discussed above,
eccentricity of the coil is measured as the distance between center of
the magnetic flux field and center of the coil. Note again FIGS. 1B, 1C
and 5. The eccentricity of the coil may be utilized for determining the
position of the center of the moving coil (Step 106).

[0072] In assembly/manufacturing relating to the geophones, the location
of the moving coil center may be used for purposes such as modifying
assembly parts, adjusting assembly jigs, QC of fabricated products, among
other applications that are known to those of skill in the art (Step
108). The information relating to the location of the moving coil center
may also be utilized for purposes relating to seismic surveying
activities (Step 110).

[0073]FIG. 4B is a schematic depiction of one exemplary technique for
determining the location of the center of a moving coil 12 of a geophone
10. As shown in the exemplary circuitry of FIG. 4B, contact points or
electrodes 30, 32 may be provided at both ends of the moving coil 12 that
are electrically connected to a pair of springs 20, 22. Electrical
signals from the moving coil 12 are output by terminal pins 34 that pass
through end caps 26 and are connected with springs 20, 22. The electrodes
32 that extend from the springs touch a corresponding contact point 30,
which is connected to the metallic housing 24, when the moving coil 12 of
the geophone touches an end cap 26. The electrode contact will be
determined by monitoring insulation between the terminal(s) 34 and
housing 24, via external electrodes 34 and housing 24. In this, the
direct current that is applied to the moving coil is monitored when the
moving coil touches an end cap.

[0074] As previously discussed above, a determination of the center of the
geophone moving coil (Step 106 in FIG. 4A) may be utilized for adjusting
assembly parameters for geophones, such as adjusting an assembly jig,
modifying assembly parts, etc., to ensure that the moving coils of the
geophones are properly centered in the magnetic fields during
fabrication. Furthermore, measurement of the center of the moving coil
for assembled geophones after fabrication provides the ability to
eliminate poorly assembled geophones during the manufacturing process.
Note again Step 108 in FIG. 4A.

[0075] Techniques according to the present disclosure relating to seismic
surveying activities (Step 110 in FIG. 4A) are schematically depicted in
FIGS. 14A and 14B and FIGS. 15A-15C, and discussed hereinafter.

[0076] As previously described above, a geophone has a moving mass
suspended in a magnetic field by means of spring(s) as shown in FIG. 1B.
When the geophone is vertical, the moving coil is displaced by the force
of gravity, i.e., the natural displacement due to gravity x0. Note
FIG. 5. In this, due to pre-stress in the moving coil spring a vertical
type geophone will have no natural displacement due to gravity whereas a
geophone without pre-stress in the spring will have natural displacement
as depicted in FIG. 5. For a low frequency vertical geophone, the spring
is pre-stressed so that the moving coil is located in the center of the
magnetic field. A geophone configured or designed according to these
principles does not function when it is horizontal since the pre-stressed
spring brings the coil to the upper end cap. On the other hand,
omni-tiltable geophones have higher natural frequencies to reduce the
amount of natural displacement so that the natural displacement of the
coil is in the working range for all angles of tilt.

[0077] When a geophone without pre-stress in the spring, for example, an
omni-tiltable geophone, is vertical, the gravity force acting on the
moving coil is F=mg, where m is the mass of the moving coil and g is the
gravitational acceleration. Note FIG. 1B. A force F that is required to
displace the coil is F=kx, where k is the spring constant and x is the
position of the moving coil relative to the neutral or natural position
x0. Since gravity force is balanced with the force applied by the
spring,

kx = mg or x = mg k . Since
ω 0 = k m , Equation 1 ##EQU00002##

Equation 1 can be rewritten as:

x 0 = - g ω 0 2 ##EQU00003##

where ω0 is the angular natural frequency defined as
ω0=2πf0 and f0 is the natural frequency of the
geophone. If the geophone is tilted by θ, measured from the
vertical, the natural displacement is:

x 0 = - g ω 0 2 cos ( θ ) Equation
2 ##EQU00004##

Determination of Tilt

[0078] In aspects of the present disclosure, tilt of a geophone may be
determined by using a step test. In this, the present techniques are
improvements over conventional geophone tilt determination methods since
other external sensors, such as tiltmeters, are not required. A direct
electrical current that is sufficient to lift and/or lower the moving
coil of a geophone is applied to the moving coil so that the moving coil
is displaced from its neutral position x0 to the top and/or bottom
end cap of the geophone. The direct electrical current is abruptly
removed so that the geophone outputs a voltage that is proportional to
the initial position. Note FIG. 5. For example, the stroke of a 18 Hz
geophone is 1.5 mm and the natural displacement, i.e., the displacement
due to gravity, is about 0.76 mm when the geophone is vertical. If the
moving coil is initially lifted to the top, then the moving coil is moved
by 1.5+0.76=2.26 mm. If the coil is pulled down to the bottom, then the
moving coil is moved by 1.5-0.76=0.74 mm.

[0079] The present inventors have found that the natural displacement of a
moving coil is a function of the tilt of the geophone. The inventors have
conceived the idea of geophone tilt determination utilizing a step test
in which the moving coil is dislocated to the upper end cap of the
geophone housing by applying a voltage that is sufficiently high to lift
the moving coil to the maximum position. As shown in FIG. 5, the
displacement of the moving coil is no longer a function of applied direct
electrical current. The direct electrical current is then released so
that the moving coil experiences natural, i.e., free, oscillation or
vibration.

[0080] The geophone step response may be expressed in geophone parameters,
open circuit sensitivity So, damping factor Do, and natural
frequency fo, and the distance x1 that the coil is lifted to
the upper end cap, as

By measuring the amplitude epl, distance xl that the coil is
lifted to the upper end cap can be determined by Equation 12. As
discussed above, the distance xl is relative to the center of the
moving coil.

Temperature Effects

[0083] The present inventors have noted that there are temperature effects
in determining geophone tilt using a step test. In this, the inventors
realized that the geophone response parameters So, Do, and
fo change with temperature changes.

[0084]FIG. 7 shows the temperature dependence of the geophone response
parameters So, Do, and fo for one type of geophone using
test data. In this, while calculating the moving coil position or the
angle of tilt, the geophone response parameters may be calibrated at
working conditions, i.e., in-situ.

[0085] Alternatively, it is also possible to predict the geophone response
parameters by measuring the temperature. In this, the DC resistance
(DCR), R, of the moving coil changes with temperature, for example, for a
copper (Cu) wire, as follows:

R=R(20)*(1+0.00393DT) Equation 13

[0086] Where R(20) is a nominal resistance measured at 20 degrees
centigrade and DT is the temperature difference from the reference
temperature, 20 degrees centigrade. Therefore, it is possible to measure
the temperature of a geophone by measuring the moving coil resistance.
Then the geophone response parameters can be predicted from the
temperature curves as shown in FIG. 7. The aforementioned principles
relating to DCR and temperature are described in more detail in the prior
U.S. patent application Ser. No. 11/733,214, previously incorporated
herein by reference.

[0087] The natural frequency fo, open circuit damping Do, and
open circuit sensitivity So, for one type of geophone shown in FIG.
7, may be defined as:

f0(T)=f0(20)*(6.4082e-8*DT2-9.4429e-5*DT+1) Equation 17

D0(T)=D0(20)*(2.9535e-6*DT2-0.0026*DT+1) Equation 18

S0(T)=S0(20)*(-7.2594e-7*DT2-2.7676e-4*DT+1.) Equation 19

[0088] To avoid calibration and compensation errors, it is possible to
eliminate D0 and S0 from geophone tilt determination. By
displacing the geophone moving coil to the bottom end cap of the
geophone, the response is written as:

It is noted that Equation 3 was a step down response from the upper end
cap of the geophone and Equation 20 is a step up response from the lower
end cap of the geophone. Both responses are simulated and shown in FIG.
8.

[0089] By dividing e1 by e2, the time response terms are
cancelled and the ratio is simply reduced to the ratio of traveling
distances as:

e1 and e2 may be determined at any point in the geophone step
responses. For example, the first peak and trough; the second, third peak
and trough or even later; an integration of half cycle up to the first
zero crossing; the integral of absolute values over the same time period.
To determine the coil eccentricity δ, only the number representing
the ratio of e1 and e2 is needed.

[0091] The coil eccentricity δ is related to the angle of tilt as:

δ - δ 0 = g ω 0 2 cos θ
Equation 24 ##EQU00018##

where δ0 is the coil position value when a geophone is in a
horizontal position. Ideally, δ0 is zero; however, in reality
δ0 will be a finite number due to manufacturing tolerances. It
is also noted that the coil eccentricity δ is a function of the
voltage responses for the step down and step up tests. The angle of tilt
is determined from the moving coil eccentricity and spring constant or
natural frequency f0 that is least sensitive to temperature, as
shown in FIG. 7. Step Test with Imaginary Short

[0092] As previously mentioned above, a GAC is a geophone that is
configured or designed for specific applications. In this, a GAC is
usually used in an imaginary short fashion. Note FIG. 9. Further
description relating to the device of FIG. 9 may be found in Japanese
Patent No. P3098045. The negative feedback makes the input to zero volt
and the input impedance is zero. If a geophone is connected to such an
operational amplifier with an imaginary short, as shown in FIG. 9, the
geophone behaves as an accelerometer. In this case, the total damping
factor becomes:

D = D 0 + S 0 2 2 rm 0 ω 0 Equation
25 ##EQU00019##

and the step responses from the top and the bottom end caps are written
as:

It should be noted that {square root over (1-D2)} becomes an
imaginary number for D>1. Since the ratio of the step responses is
reduced in the same fashion as Equation 21 above, the results, i.e.,
Equations 22, 23 and 24 above, are still valid.

[0093] FIG. 10 shows simulated step responses based on the parameters
shown below with over damping using the imaginary short circuit shown in
FIG. 9.

f0=18 [Hz]

S0=79 [V/(m/s)]

D0=0.36

[0094] m0=0.00314 [kg] r=1500 [ohm] xm=2.8/2 [mm] R=100000 [ohm]

[0095] The present inventors considered the question as to what will
happen if a step test causes collision of the moving coil at the bottom
end cap. FIG. 11 shows the result of such a collision. The electrical
output is proportional to the velocity of the moving coil. The trough
occurs when the moving coil passes the center of maximum excursion. Then,
the moving coil hits the bottom end cap of the geophone. As previously
described above, e1 and e2 may be determined at any point of
the step responses, and a characteristic amplitude can be determined at
the trough even if there is a collision of the moving coil afterwards.

[0096]FIG. 12 shows an example of step down and step up responses
measured sequentially using a geophone. The amplitudes are determined at
the first trough and the first peak. FIG. 13 shows step down and step up
responses for different angles of tilt using measured data. The
amplitudes of the step down and step up responses are similar when the
angle of tilt is zero (measured from a horizontal position). The step
down amplitude is higher than the step up amplitude when angle of tilt
increases.

[0097] FIGS. 14A and 14B depict exemplary techniques in accordance with
the present disclosure for land seismic survey. FIGS. 15A-15C depict some
exemplary techniques in accordance with the present disclosure for seabed
seismic and borehole seismic surveys.

[0098] For land seismic survey, the geophone is assumed to be in a
vertical orientation (note FIG. 1A). In this, the angle of tilt (θ)
measured from the vertical is to be determined, as shown in FIG. 14A.
FIG. 14B shows exemplary techniques for land seismic surveying wherein
the eccentricity of a moving coil may be determined using the techniques
herein. As previously discussed above, the techniques of the present
disclosure may be utilized for determining tilt of a geophone, for
example, a geophone that is planted in the ground for land seismic. In
this, quality control (QC) of the geophones planted in the ground may be
performed based on the determined eccentricity of the moving coils of the
geophones to see if the geophones are vertically planted, or are planted
within the tilt tolerances for seismic data acquisition by use of the
geophones. In such a situation, it would be possible to inform the person
or persons who planted the geophones to replant them so that the
geophones are properly planted in the ground for purposes of land seismic
survey. For example, the person or persons who planted the geophones may
perform the testing during the geophone planting operation to ensure that
the geophones are properly planted. Such QC information may be used to
verify the quality of the geophone planting operation providing a client
with a valuable answer product/service.

[0099] As depicted in FIG. 14B, geophones need to be calibrated at their
vertical orientation. For example, prior to planting geophones may be
mounted on a calibration block (schematically depicted in FIG. 14B) that
is suspended in a horizontal position. The natural displacement of the
moving coil(s), including any manufacturing tolerance, may be determined
at their vertical position. After the geophones are planted for land
seismic, step tests may be conducted, and using in-situ calibration or
measured DC resistance (DCR), geophone response parameters S0,
D0, and f0, may be determined, as previously discussed above.
Alternatively, or in addition, both step up and step down tests may be
conducted. The data may be used to determine the moving coil position, as
discussed above.

[0100] Table 1 below shows the experimental results for a geophone used in
land seismic. The first peak amplitude was measured from step test data.
The moving coil eccentricity was calculated from the peak measurements.
When tilt is zero, the natural displacement is -0.7373 mm, while the
theoretical value that is calculated based on f0=18 Hz is 0.7662 mm.
The calculated results are graphically shown in FIG. 14A. It is noted
that the above discrepancy between the calculated and theoretical values
may be due to tolerance of f0 and/or non-linearity of the spring. To
calculate the tilt angle (θ) from the measured signal, the data is
calibrated so that the moving coil eccentricity is the same as the
theoretical number, as shown in the column to the right of coil
eccentricity in Table 1 below. The angle of tilt was estimated by using
Arc SIN(δ/(ω02/g)) based on a nominal natural
frequency of 18 Hz. The calculated tilt is only a few degrees off from
the actual tilt of the geophone.

[0101] It is noted that there are some residual coil centering errors in
the geophones used for the experiments which are less than 0.1 mm;
however, the error is finite. To improve the determination of coil
eccentricity, the residual coil centering errors can be calibrated by
maintaining the sensor in a horizontal orientation, a measured value for
f0 may be used, and spline function may be used to fit the non
linear behavior. For land seismic applications, the maximum tilt is 90
degrees, i.e., the geophone is horizontal, and a simple calculation of
sinusoidal function may be good enough for quality control purposes. Even
if the geophone is replanted, it is not possible to control the
orientation within a few degrees of tilt.

[0102] For OBC and borehole seismic surveys, the geophones are designed
for use in horizontal, vertical or upside down orientation (note FIGS.
2A, 2B, 3A and 3B). In this, the relative bearings of two components of
the 3-component geophone package have to be determined, as shown in FIGS.
2B and 3B. As previously discussed above, the techniques of the present
disclosure may be utilized for determining relative bearing of a geophone
package, for example, a geophone package that is deployed at the seabed
or in a borehole for seismic survey.

[0103]FIG. 15C shows exemplary technique's for seabed and borehole
seismic surveying wherein the tilt of a moving coil may be determined
using the techniques that are disclosed herein. In this, relative bearing
of a seismic sensor package deployed in a borehole or at a surface of a
seabed may be determined based on the tilt of the geophone(s) in the
package.

[0104] For borehole seismic (note FIGS. 3A and 3B), the two horizontal
sensors X and Y are horizontal when the tool is vertically positioned.
The tilt of the vertical geophone Z is the same as the deviation of the
well. In this, the tilt angles of the two horizontal geophones X and Y
are to be determined so as to find the orientation (relative bearing) of
the sensor package in the well, as shown in FIG. 3B. The tilt angle
(θ) is measured from the vertical axis when the geophone is in a
horizontal position, as shown in FIG. 15A.

[0105] The coil eccentricity is calculated from test data shown
graphically in FIG. 15A, and is tabulated in Table 2 below for different
tilt angles (shown in FIG. 15A) for the horizontal geophone.

[0106] The results are also shown in the graph of FIG. 15A. It is seen
that the moving coil is not in the center position when the tilt is zero.
This is due to fabrication errors in the test geophone. It is also seen
that the measured coil eccentricity is slightly larger than the
theoretically calculated coil eccentricity based on the nominal natural
frequency. The error may be due to 1) the actual f0 may not be the
same as the nominal f0, and 2) an extra force is required to
overcome the non-linearity of the spring to fully displace the coil to
the end of the housing.

[0107] For a downhole seismic tool and seabed sensor package, it is
possible to perform a calibration when the sensor package is horizontal.
The positions of the geophones in the sensor package are changed by
rotating the sensor package. The measured moving coil eccentricity at
zero tilt is removed from all the measured coil positions for different
sensor orientations (see column Offset in Table 2 above). The measured
moving coil eccentricity is calibrated to the coil eccentricity measured
at the vertical and up side down positions (see column Amplitude in Table
2 above). The calibrated coil eccentricity is also shown in FIG. 15A.

[0108] The present inventors have also used a polynomial expansion instead
of a sinusoidal function. Since the angle of tilt and moving coil
eccentricity measurements are in a one-to-one relationship, it is
possible to express the angle of tilt by a polynomial function of moving
coil eccentricity instead of a sinusoidal function. As seen in FIG. 15B,
the error for the tilt obtained from sinusoidal function was a few
degrees different from the measurements. A large error for a large tilt
is acceptable for land seismic applications, since such geophones are not
supposed to be used horizontally, and the QC is meant to find such poorly
planted geophones. For seabed and borehole seismic, the geophones can be
tilted by any angle, and the amount of the tilt is the information of
interest to find the direction of seismic wave propagation. It is
possible to model the non-linearity by high order sinusoidal functions,
such as

[0109] First, calibration data are obtained when the sensor is vertical,
horizontal, and upside down. Then, the value when the sensor is
horizontal is subtracted. The relationship between normalized coil
eccentricities and tilt angles is expressed by a single fifth order
polynomial. A polynomial function was obtained by curve fitting test data
from two geophones and is:

tilt=2.606e-12p5+4.073e-7p3-0.01197p Equation 28

where tilt is in degrees and p is the normalized moving coil
eccentricity. It is seen in FIG. 15B that polynomial fitting matches with
measured data better than a sinusoidal function. FIG. 15C shows a
calibration process of the sensor package for borehole seismic and seabed
seismic. By rotating the sensor package, two horizontal geophones for
borehole seismic and one vertical geophone and one horizontal geophone
for seabed seismic are calibrated every 90 degrees. FIG. 15C also depicts
two possible methods for determining tilt of geophones in a borehole or
at a seabed.

[0110] FIGS. 16A and 16B depict some exemplary techniques in accordance
with the present disclosure for determining relative bearing of sensor
packages in seabed and borehole seismic, respectively.

[0111] In a sensor package that is attached to an OBC cable, the geophones
at the Z-axis and X-axis can rotate when the OBC cable is deployed. If
the Z-axis geophone is tilted by θ degrees, the X-axis geophone
will be tilted by θ+90 degrees. The amount of displacement or
eccentricity of the moving coil of a geophone in the sensor package is
determined by the relative bearing of the sensor package, as shown in
FIG. 16A for seabed seismic applications.

[0112] It is possible to determine the tilt of a geophone in a sensor
package by measuring the coil displacement or eccentricity of either the
Z-axis or X-axis geophone. If both are used, it is possible to find out
whether the sensor package is tilted in the direction of the X-axis
geophone, i.e., the relative bearing of the sensor package.

[0113] In borehole seismic on the other hand (note FIG. 16B), if the well
is vertical it is not possible to find the relative bearing of the sensor
package. Typically, a borehole has some deviation, and the well
trajectory may be determined by other methods such as, for example,
measurement-while-drilling or a gyroscope survey.

[0114] Since the tilt of the Z-axis geophone is the same as the deviation
of the well (note again FIG. 3B), it is not necessary to determine its
tilt although it may be desirable to cross check the measurements.
However, since the sensor package orientation in the borehole is unknown,
and as discussed above sensor packages tend to be cylindrical and prone
to rotating in the borehole, the relative bearing along the X-axis and
Y-axis needs to be determined.

[0115] If the well deviation is finite, the moving coil displacements of
the X-axis and Y-axis geophones are:

[0116] The Y-axis geophone is rotated by 90 degrees relative to the X-axis
geophone in a right hand coordinate system (note FIG. 3B), and the
corresponding coil displacements or eccentricities are graphically shown
in FIG. 16B for different angles of tilt of the sensor package.
Accordingly, it is possible to determine the orientations, i.e., relative
bearings, of the X-axis and Y-axis by measuring the coil displacements or
tilt of the corresponding geophones.

[0117]FIG. 17 is a schematic depiction of one exemplary technique for
adjusting the position of a moving coil 12 of a geophone 10 to correct
for coil eccentricity. An adjustment mechanism may be built into a
geophone for purposes of adjusting or correcting coil eccentricity as
shown in FIG. 17. The mechanism of FIG. 17 has a screw 40 and shaft 42
associated with a spring holder 21 of the moving coil spring 20. By
adjusting the screw 40 the shaft 42 moves the spring holder 21, the
spring 20 and the associated moving coil 12 so that moving coil
eccentricity can be corrected. In this, the techniques described herein
may be used to test a geophone for coil eccentricity. When coil
eccentricity is observed, for example, during the manufacturing process,
the moving coil may be adjusted using a device such as depicted in FIG.
17, for example, to correct the eccentricity after fabrication of the
geophone.

[0118] Generally, the techniques disclosed herein may be implemented on
software and/or hardware. For example, they can be implemented in an
operating system kernel, in a separate user process, in a library package
bound into network applications, on a specially constructed machine, or
on a network interface card. In one embodiment, the techniques disclosed
herein may be implemented in software such as an operating system or in
an application running on an operating system.

[0119] A software or software/hardware hybrid implementation of the
present techniques may be implemented on a general-purpose programmable
machine selectively activated or reconfigured by a computer program
stored in memory. Such a programmable machine may be implemented on a
general-purpose network host machine such as a personal computer or
workstation. Further, the techniques disclosed herein may be at least
partially implemented on a card (e.g., an interface card) for a network
device or a general-purpose computing device. Referring now to FIG. 18, a
network device 60 suitable for implementing various aspects of the
present techniques includes a master central processing unit (CPU) 62,
interfaces 68, and a bus 67 (e.g., a PCI bus). When acting under the
control of appropriate software or firmware, the CPU 62 may be
responsible for implementing specific functions associated with the
functions of a desired network device. For example, when configured as a
general-purpose computing device, the CPU 62 may be responsible for data
processing, media management, I/O communication, calculating the geophone
moving coil eccentricity, calculating the geophone response parameter
values, performing geophone tilt determination operations, etc. The CPU
62 preferably accomplishes all these functions under the control of
software including an operating system (e.g. Windows XP), and any
appropriate applications software.

[0120] CPU 62 may include one or more processors 63 such as a processor
from the Motorola or Intel family of microprocessors, or the MIPS family
of microprocessors. In an alternative embodiment, processor 63 is
specially designed hardware for controlling the operations of network
device 60. In another embodiment, a memory 61 (such as non-volatile RAM
and/or ROM) also forms part of CPU 62. However, there are many different
ways in which memory could be coupled to the system. Memory block 61 may
be used for a variety of purposes such as, for example, caching and/or
storing data, programming instructions, etc. The interfaces 68 are
typically provided as interface cards (sometimes referred to as "line
cards"). Generally, they control the sending and receiving of data
packets over the network, and sometimes support other peripherals used
with the network device 60, such as, for example, display devices 70
and/or printing devices 72. It will be appreciated that the various
techniques of the present disclosure may generate data or other
information to be presented for display on electronic display devices
and/or non-electronic display devices (such as, for example, printed for
display on paper).

[0121] Examples of other types of interfaces that may be provided are
Ethernet interfaces, frame relay interfaces, cable interfaces, DSL
interfaces, token ring interfaces, and the like. In addition, various
very high-speed interfaces may be provided such as fast Ethernet
interfaces, Gigabit Ethernet interfaces, ATM interfaces, HSSI interfaces,
POS interfaces, FDDI interfaces and the like. Generally, these interfaces
may include ports appropriate for communication with the appropriate
media. In some cases, they may also include an independent processor and,
in some instances, volatile RAM. The independent processors may be used,
for example, to handle data processing tasks, display tasks,
communication tasks, media control tasks, etc.

[0122] Although the system shown in FIG. 18 illustrates one specific
network device, it is by no means the only network device architecture on
which the present disclosure can be implemented. For example, an
architecture having a single processor that handles communications as
well as routing computations, etc. is often used. Further, other types of
interfaces and media could also be used with the network device.
Regardless of the network device's configuration, it may employ one or
more memories or memory modules (such as, for example, memory block 65)
configured to store data, program instructions for the general-purpose
network operations and/or other information relating to the functionality
of the techniques described herein. The program instructions may control
the operation of an operating system and/or one or more applications, for
example. The memory or memories may also be configured to store data
structures, seismic logging information, geophone response parameter
information, prospecting information, and/or other specific non-program
information described herein.

[0123] Because such information and program instructions may be employed
to implement the systems/methods described herein, the present disclosure
also relates to machine readable media that, include program
instructions, state information, etc. for performing various operations
described herein. Examples of machine-readable media include, but are not
limited to, magnetic media such as hard disks, floppy disks, and magnetic
tape; optical media such as CD-ROM disks; magneto-optical media such as
optical disks; and hardware devices that are specially configured to
store and perform program instructions, such as read-only memory devices
(ROM) and random access memory (RAM). The present disclosure may also be
embodied in a carrier wave traveling over an appropriate medium such as
airwaves, optical lines, electric lines, etc. Examples of program
instructions include both machine code, such as produced by a compiler,
and files containing higher level code that may be executed by the
computer using an interpreter.

[0124] The various aspects of the disclosure were chosen and described in
order to best explain the principles of the invention and its practical
application. The preceding description is intended to enable those of
skill in the art to best utilize the invention in various embodiments and
aspects and with modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be defined
by the following claims.

Patent applications by Francis Maissant, Oslo NO

Patent applications by SCHLUMBERGER TECHNOLOGY CORPORATION

Patent applications in class TESTING, MONITORING, OR CALIBRATING

Patent applications in all subclasses TESTING, MONITORING, OR CALIBRATING